Sainsmart mechanical end stop = Blown mega

Hey All,
So I finally am finishing up my Prusa I3. With sainsmart Ramps 1.4 + A4988 + Mega2560 R3 + LCD 12864.
I was able to get everything working but the end stops. I ended up shorting VCC to Gnd hooking up the end stop.
I have since found this helpful link for correct hook up [forums.reprap.org] .
So now when i turn on my 12v my LCD goes dark. I can still see the menu and all my functions work but it's not right.
I tested the ramps D1 and D2 and they seem ok. From what I see on the ramps schematic. The VCC comes from the mega and I shorted it to ground. With 12v supplying the mega via VIN.
So I'm leaning towards the mega as the problem.
My Ramps has both D1 and D2 installed. I would like to be able to run a project via usb and host, as well as put a project on the sd card and run it stand alone. So I believe the D1 is needed.
Has anyone seen this ?

Thanks

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"To teach is to learn twice"

when you plug endstops in wrong way you short +5 to gnd and blow the voltage regulator on the mega

The ramps supplies the voltage regulator 12v via D1 which it converts to 5v for both the mega and the ramps.

D1 is require to power the mega if it is not connected to usb.. but usb alone may not be enough to power the lcd as well...

The voltage regulator on the Mega is short-circuit and over-temperature protected, so it's unlikely to have failed. Also if you can see the LCD menu and the functions work, then Vcc must be OK, unless you only get the LCD menu when you apply USB power. So it sounds to me that it is just the backlight on the LCD that is not working. Can you confirm this? Do you have a multimeter that you can use to measure the 5V rail when just 12V power is provided and the USB cable is disconnected?

12864 displays vary a lot and I suspect many of the ones sold on eBay are rejects, albeit ones that mostly work. Some include the backlight series resistor, but others do not. So you may have blown your 12864 backlight by connecting it directly to 5V without a series resistor. Alternatively, you might just have a bad connection to one of the backlight pins on the 12864. Use a multimeter to easure the voltage between the backlight connections at the side of the 12864 (these are typically marked A and K), to see if voltage is getting there.

Bear in mind that the backlight on a 12864 needs a lot of current (about 100mA AFAIR) and this will make the regulator on the Mega run warm or hot. You can avoid this by powering the LCD backlight from 12V via a series resistor instead of from 5V. 100 ohms rated at 1W will give you about 80mA backlight current.

Edited 1 time(s). Last edit at 12/03/2014 08:11AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

PS - it occurs to me that when you say the LCD goes dark, you may mean that the backlight lights up but all the pixels are dark. If that is the case, then the contrast pot or resistor on the 12864 needs adjusting, or you need to add an external contrast pot. Some 12864s have a contrast pot on the back of the board, others have fixed resistors, and yet others bring then pin out to the edge connector so that you can connect an external pot. I would need to see a photo of the back of the 12864 to tell which sort you have.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for the replies guys.
If I just have the usb plugged in all works fine and the lcd works perfect. When I power up my 12v the lcd greys out. I can still see the menu but it's washed out. I can still run all the functions and that's what perplexes me. I'm leaning twards the mega. I think your both right. I blew the mega voltage reg and it's not supplying the proper voltage or current to the lcd. So when I just use the usb 5v lcd is fine.
How does the mega switch between usb supply and external ?
I have another mega I'm going to try. Are there any tests I can do before I plug in the other mega?

Thanks again as you can tell my electronics is less than stellar.

1. Before you had the short, did the LCD display work properly without the USB connected, or did you never test it that way?

2. Do you have a multimeter? If so, measure the voltage on the +5V rail with and without the USB connected, and with and without the LCD connected.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Yes dc42 it worked fine with just the USB, both USB and 12v, and 12v only.

It's strange. I swapped a mega from another project I have and all is good !!!
So it was the mega. It worked out good because the burnt up mega works fine connected to USB only and that's how I'm using it in the other project.
Now I have end stops with only 3 wires (I soldered the 2 gnds together at the sw and clipped the other gnd wire). So late one night I don't make the same mistake.

Thanks to everyone.

Glad you got it all sorted

Dispite dc42 saying "The voltage regulator on the Mega is short-circuit and over-temperature protected, so it's unlikely to have failed. " this proves to me it was yet annother dead voltage regulator on the mega..

I have see them die alot, nearly every time someone shorts the +5 to gnd via a endstop switch.

I agree, those regulators never seem to survive a short like that

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Waitaki 3D Printer

Yes, I agree that the regulator was at fault. I guess there must be some other factor in play here, such as the nature of the wiring causing a transient that the regulator can't survive.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].